Jennie Formby, an organiser for the Unite union, appears almost certain to clinch the post.
Photograph: David M. Benett/Getty Images for Grazia Magazine

Jennie Formby, an organiser for the Unite union, looks all but certain to be Labour’s next general secretary, completing Jeremy Corbyn’s control of the most senior posts in the party, after the GMB union threw its weight behind her.

Formby’s candidacy has been keenly promoted by Corbyn’s office, and supportive Labour MPs include Chris Williamson, and party chair Ian Lavery.

But other trade unions were initially concerned at what they regarded as a Unite stitch-up – and Jon Lansman, leader of the pro-Corbyn grassroots campaign group Momentum, put himself forward, complaining about “machine” politics inside Labour.

Lansman withdrew his name on Sunday and, with the GMB’s backing, Formby appears almost certain to clinch the post, which is voted on by members of the party’s ruling national executive committee.

In a statement, the GMB’s general secretary, Tim Roache, said: “This isn’t a small decision, we’ve taken time as a union to discuss it, to think about where Labour needs to be.

“GMB members first and foremost need a Labour government, and we need our party to understand us. Jennie has been a trade unionist for decades. I have no doubt she understands the challenges and issues working people face and that she’ll do everything she can to put them at the heart of Labour’s campaign for government.”

The post became vacant after the party’s longstanding general secretary Iain McNicol announced his departure.

McNicol was controversial with many Corbyn supporters because of his role in the row over which party members could vote in the 2016 Labour leadership contest; but he worked closely with the leaders’ office during last year’s general election.

Another high-profile departure was announced on Tuesday, with Emilie Oldknow, one of the party’s executive directors, emailing staff to say she would be “leaving in the summer to pursue some new and exciting opportunities”.

One senior party figure said: “She ran the Labour party, Iain was the general secretary in name only – her departure is far more significant than Iain’s.”

There have been a series of resignations in recent months as Corbyn’s team continue transforming Labour party machinery.